TRANSPORT AND METABOLISM OF POLYAMINES IN WILD AND MULTIDRUG-RESISTANT HUMAN LEUKEMIA (K-562) CELLS

Multidrug resistance (MDR) can be defined as the resistance of cancer cells not just to chemotherapeutic agents to which they have been expo sed but also to other apparently unrelated compounds. This MDR phenoty pe is commonly associated with the high expression of levels of 170 kD a P-glycoprotein, encoded by MDR genes. In the present study, the upta ke kinetics of polyamines and their biosynthesis were studied in wild and multidrug resistant (MDR) K 562 cells in culture. The rate (V-max) of polyamine uptake was significantly lower in MDR cells than that in wild type cells, whereas the K-m for the uptake was not significantly different in these cells, suggesting that polyamine transporter is no t modified in MDR cells, though their different physiological state in fluences the uptake process. In a 32 h chase, the transported radioact ive polyamines were gradually interconverted. [C-14]putrescine was con verted into [C-14]spermidine following between 15 min and 32 h of cult ure, and into [C-14]spermine after 16 h of culture, in both the cell t ypes; however, the levels of interconverted radioactive polyamines wer e always lower in MDR cells as compared with wild type cells. Similarl y, internalized [C-14]spermidine was converted into [C-14]spermine, bu t not into [C-14]putrescine in both the cells types. [C-14]spermidine is metabolized into [C-14]spermine after 4 h of culture in wild type c ells, whereas in MDR cells the interconversion of [C-14]spermidine int o [C-14]spermine is seen only after 16 h of culture. Blocking of the t ransmembrane drug efflux pump, expressed in the MDR cells, by preincub ation in the presence of verapamil, did not influence the uptake of ei ther of the two polyamines (putrescine and spermidine) by MDR cells. O n the contrary, this kind of preincubation of wild type cells in the p resence of verapamil significantly increased the uptake of these two p olyamines. The levels of intracellular polyamine contents in MDR cells were always lower than those in the parental cell line. These results demonstrate that MDR cells are defective in both the uptake of polyam ines and their biosynthesis as compared with wild type cells.